This is Ciara from the Content API support team as well. Allow me to assist.
Upon checking the Content API feed on your account, it appears that the products you are trying to add for the vehicle ads is uploaded on an online channel. As you may have referred to this sample JSON request, the products need to be uploaded on a local channel for the vehicle ads program.
Moving forward, kindly refer to the provided sample JSON request and you can include on the request the included_destinations with the value of VehicleInventoryAds. If you encounter any issues, kindly provide us with the Complete API request and response logs in JSON format when you tried to submit your products to a local channel for the vehicle ads program but encountered an error.
You can provide it via the Reply privately to author option. If this option is not available, please send this requested information through our email <shopping-a...@google.com> instead for private purposes. WIth that said, your previous forum response will be deleted as it contains private information.
Let me know if you have any questions.